Check In Selective Breeding Lab

Arikara, Nesaru & The Corn Mother

Where Did Corn Come From?

Origins Of Modern Corn 4,000 BCE Teosinte, Mexico Corn does not reach U.S. until 3,000 BCE (New Mexico) and midwest (Ohio) in 1,700 BCE

Domestication of Corn Brought to Europe in 1490’s by Columbus First Anglo-Saxon domestication in 1621 via Pawtuxet Tribe in Plymouth, MA

How Have Corn Phenotypes Changed As A Result of Domestication?

Corn Phenotypes Sugar Content –Starchy (plump) –Sweet (shriveled) Color –Purple –Yellow

Modern Corn Through intensive hybridization, modern corn is a combination of yellow and starchy phenotypes Both yellow and starchy phenotypes were rare when corn was first domesticated

Hybridization To eliminate the “undesirable” traits of purple and sweet, corn growers have bred individuals showing the desired phenotypes together until pure breeding lines are produced

Determining Genotypes From Ratios of Progeny Each kernel on the cob represents is a unique genetic individual resulting from a separate fertilization By examining the ratios of progeny phenotypes, the parental genotypes that produced the cob may be determined

If we know the parental genotypes, why do some progeny not ascribe exactly to the expected phenotypic ratio?

Barbara McClintock Barbara McClintock studied ratios of corn phenotypes and noted that often the ratios did not match the expected outcomes Percent Deviation

Jumping Genes Through examination of the percent deviation McClintock hypothesized that genes could “jump” from one chromosome to another. If genes jump to the wrong location, they are not expressed properly The higher the percent deviation from the expected phenotypic ratio, the closer the genes were on the chromosome

Transposons (i.e. “Jumping Genes”) DNA sequences that can cut and paste themselves into other locations in the genome Their placement may cause the transposed element not to be expressed OR to inhibit the expression of a pre- existing sequence of DNA Transposons that impact promoter regions are most likely to see differences in expression

How much influence do transposons have on the expression of color and sugar content phenotypes in domesticated corn?

Lab: McClintock Corn Analysis Purpose: To determine the effect of transposons on the phenotypic expression of color and sugar content in domesticated corn Method: Calculation of percent deviation between expected and actual phenotypic ratios.

Protocol 1: Assessing the Progeny By Phenotype Obtain an ear of corn and write the letter (corn code) in your lab notebook Count and classify each of the kernels into one of four phenotypic classes –Purple Starchy –Purple Sweet (shriveled) –Yellow Starchy –Yellow Sweet

Protocol 2: Determining Parental Genotypes Sum the subtotals for a total # of kernels/progeny Divide through each of the phenotypic subtotals by the smallest subtotal to obtain a ratio Determine which two-trait phenotypic ratio your data models Based upon this ratio, work backwards to determine the genotypes of the parents that produced the kernels on your cob

Protocol 3: Determining Expected Phenotypic Outcomes Turn your ratio into fractions (i.e. 9:3:3:1 = 9/16ths dom/dom, 3/16ths dom/rec, etc.) Multiply the total number of kernels/progeny by each of these fractions separately to determine the number of kernels that should have been that phenotype. This is your EXPECTED value for that phenotypic category. Repeat for all four of the categories

Protocol 4: Calculating Percent Deviation For each of the four phenotypic categories, find the absolute value difference between the expected # of progeny and the actual # of progeny that exhibited this phenotypic combination Repeat for all four phenotypic categories and SUM the deviations. Divide the total sum of deviations by the total number of kernels. This is your percent deviation, which suggests how much influence transposons had on your population